Polyesters, especially polyethylene terephthalates (PET), possess good mechanical properties, e.g., tensile strength, elasticity and impact strength, as well as good chemical stability, heat resistance and electrical insulation property; therefore, they have been widely used in various applications such as capacitors, packaging materials, graphics and magnetic recording media.
For use in such applications, polyester films are generally subjected to successive biaxial extension and thermal treatment processes. The biaxially oriented film treated thermally by a conventional method has a significant disadvantage, in that the film has a poor levelling property due to residual extension and contraction stresses applied during the thermal treatment process. The poor levelling property of the film entails poor appearance, coatability and windability, which causes blurring of prints and also curling of the film when laminated.
Therefore, numerous attempts have been made to ameliorate such problems. For example, Japanese Patent Publication No. 72-3196 discloses a process for treating the portion having poor levelling property using a separate thermal relaxation apparatus. However, this method has some disadvantages, e.g., the relaxation apparatus is not easily adapted to an existing system, and the difficulty in continuous monitoring and mending defective portions when it is installed in a continuous film production system.
U.S. Pat. No. 4,293,508 teaches a method for minimizing the time needed for longitudinal extension of the film. This method still requires the use of complicate equipments that are difficult to harmonize with the existing film production line.